The present invention relates to a hydraulic drive system for a construction machine, such as a hydraulic excavator, in which load sensing control is performed to hold a delivery pressure of a hydraulic pump higher than a maximum load pressure of a plurality of actuators by a target differential pressure, and in which differential pressures across a plurality of directional control valves are each controlled by a pressure compensating valve. More particularly, the present invention relates to a hydraulic drive system in which a target compensated differential pressure of each pressure compensating valve is set by a differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators, and the target differential pressure in the load sensing control is variably set depending on an engine revolution speed.
A hydraulic drive system, in which load sensing control is performed to hold a delivery pressure of a hydraulic pump higher than a maximum load pressure of a plurality of actuators by a target differential pressure, is called a load sensing system (hereinafter referred to also as an xe2x80x9cLS systemxe2x80x9d). Usually, in the LS system, differential pressures across a plurality of directional control valves are each controlled by a pressure compensating valve so that a hydraulic fluid can be supplied to the actuators at a ratio depending on opening areas of the directional control valves regardless of the magnitude of load pressure during the combined operation in which the plurality of actuators are simultaneously driven.
In connection with such an LS system, JP,A 10-196604 discloses a hydraulic drive system in which a differential pressure (hereinafter referred to as an xe2x80x9cLS differential pressurexe2x80x9d) between a delivery pressure of a hydraulic pump and a maximum load pressure of a plurality of actuators is introduced to pressure compensating valves for setting a target compensated differential pressure of each pressure compensating valve by the LS differential pressure, and in which a target differential pressure (hereinafter referred to as a xe2x80x9ctarget LS differential pressurexe2x80x9d) in the load sensing control is variably set depending on an engine revolution speed.
By setting the target compensated differential pressure of each pressure compensating valve by the LS differential pressure, when a saturation state, where a delivery rate of the hydraulic pump is insufficient for satisfying a flow rate demanded by the plurality of directional control valves, occurs during the combined operation in which the plurality of actrators are simultaneously driven, the LS differential pressure is lowered depending on a degree of saturation, and the target compensated differential pressure of each pressure compensating valve is also reduced correspondingly. Therefore, the delivery rate of the hydraulic pump can be redistributed at a ratio of flow rates demanded by the respective actuators. Such a system is based on the concept of the invention disclosed in JP,A 60-11706.
By variably setting the target LS differential pressure depending on the engine revolution speed, when the engine revolution speed is lowered, the target LS differential pressure is also reduced correspondingly. Accordingly, even when a control lever for the directional control valve is operated in the same input amount as in the rated state, the flow rate of the hydraulic fluid supplied to the actuator is reduced and the actuator speed is slowed down. As a result, the actuator speed can be obtained corresponding to the engine revolution speed and fine operability can be improved.
Further, in connection with the LS system, GB2195745A discloses a system in which a signal pressure relief valve is disposed in a maximum load pressure line for detecting a maximum load pressure as a signal pressure, a setting pressure of the signal pressure relief valve is set to be lower than a setting pressure of a main relief valve, and the maximum load pressure having an upper limit restricted by the signal pressure relief valve is introduced to each pressure compensating valve. By providing the signal pressure relief valve in the maximum load pressure line, even when a load pressure of any one actuator reaches the setting pressure of the main relief valve and a delivery pressure of a hydraulic pump becomes equal to the maximum load pressure during the combined operation in which a plurality of actuators are simultaneously driven, it is possible to prevent all of the pressures compensating valves from being fully closed and hence prevent all of the actuators from being stopped, because the signal pressure in the maximum load pressure line is reduced to a level lower than the delivery pressure of the hydraulic pump.
However, the prior-art systems described above have problems as follows.
In the prior art disclosed in JP,A 10-196604, as described above, the LS differential pressure is introduced as the target compensated differential pressure to the pressure compensating valve. During the combined operation in which a plurality of actuators are simultaneously driven, therefore, when the load pressure of any one actuator reaches the setting pressure of the main relief valve and the delivery pressure of the hydraulic pump becomes equal to the maximum load pressure, the LS differential pressure is reduced to 0 and the pressure compensating valves are all fully closed. Consequently, no hydraulic fluid is supplied to the other actuators as well, of which load pressures do not yet reach the relief pressure, and the actuators are all stopped.
By providing the signal pressure relief valve, disclosed in GB2195745A, in the maximum load pressure line of the hydraulic drive system disclosed in JP,A 10-196604, even when the delivery pressure of the hydraulic pump becomes equal to the maximum load pressure as mentioned above, the signal pressure in the detection line is reduced to a level lower than the delivery pressure of the hydraulic pump. It is hence possible to prevent all of the pressure compensating valves from being fully closed and prevent all of the actuators from being stopped. Such an arrangement, however, causes another problem.
In the hydraulic drive system disclosed in JP,A 10-196604, the target LS differential pressure is variably set depending on the engine revolution speed. Therefore, the target LS differential pressure differs between when the engine revolution speed is set to a rated value and when the engine revolution speed is set to a lower value. The target LS differential pressure is smaller in the latter case than in the former case, and the actual LS differential pressure is also reduced correspondingly. Accordingly, if the setting pressure of the signal pressure relief valve is set to be lower than the setting pressure of the main relief valve by a value corresponding to the LS differential pressure during the rated rotation, the following problem occurs. During the rated rotation, the LS differential pressure resulting when the load pressure of the actuator is low and the main relief valve is not operated is equal to the differential pressure between the delivery pressure of the hydraulic pump and the signal pressure in the detection line resulting when the load pressure rises up to the setting pressure of the main relief valve, and hence the target compensated differential pressure of the pressure compensating valve is not changed. However, when the engine revolution speed is set to a lower value, the LS differential pressure is reduced to a level lower than that during the rated rotation as described above, while the differential pressure between the setting pressure of the signal pressure relief valve and the setting pressure of the main relief valve remains the same as the LS differential pressure during the rated rotation. Accordingly, the differential pressure between the delivery pressure of the hydraulic pump and the signal pressure in the detection line resulting when the load pressure rises up to the setting pressure of the main relief valve is larger than the LS differential pressure resulting when the load pressure of the actuator is low and the main relief valve is not operated, whereby the target compensated differential pressure introduced to the pressure compensating valve is increased. As a result, when the load pressure of any one actuator reaches the setting pressure of the main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the hydraulic fluid is supplied to the other actuators at a larger flow rate than so far, and the other actuators are sped up. Operability in the combined operation is hence remarkably impaired.
A first object of the present invention is to provide a hydraulic drive system wherein, even when a load pressure of any one actuator reaches a setting pressure of a main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the other actuators are not stopped and good operability in the combined operation is obtained.
A second object of the present invention is to provide a hydraulic drive system wherein, even when a load pressure of any one actuator reaches a setting pressure of a main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the other actuators are not sped up and good operability in the combined operation is obtained.
(1) To achieve the above first object, according to the present invention, there is provided a hydraulic drive system comprising an engine, a variable displacement hydraulic pump driven by the engine, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of directional control valves for controlling respective flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators, a plurality of pressure compensating valves for controlling respective differential pressures across the plurality of directional control valves, pump control means for performing load sensing control to hold a delivery pressure of the hydraulic pump higher than a maximum load pressure of the plurality of actuators by a target differential pressure, and a main relief valve for Restricting an upper limit of the delivery pressure of the hydraulic pump, a target compensated differential pressure for each of the plurality of pressure compensating values being set in accordance with a differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators, a target differential pressure in the load sensing control being set as a variable value depending on a revolution speed of the engine, wherein the hydraulic drive system further comprises target compensated differential pressure modifying means for setting, as the target compensated differential pressure for each of the plurality of pressure compensating valves, a modification value different from the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators, when the delivery pressure of the hydraulic pump rises up to a setting pressure of the main relief Valve.
Thus, the target compensated differential pressure modifying means is provided to set, as the target compensated differential pressure, the modification value different from the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure, when the delivery pressure of the hydraulic pump rises up to the setting pressure of the main relief valve. Accordingly, even when the load pressure of any one actuator reaches the setting pressure of the main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the target compensated differential pressure is not reduced down to 0, the pressure compensating valves are not closed, and the hydraulic fluid can be supplied to the other actuators. As a result, the other actuators are not stopped and good operability in the combined operation is ensured.
(2) Also, to achieve the above second object, according to the present invention, the modification value in the above (1) is a variable value depending on the revolution speed of the engine.
With that feature, when the engine revolution speed is lowered and the target differential pressure in the load sensing control, which is set as the variable value depending on the engine revolution speed, is reduced, the modification value set as the target compensated differential pressure is also reduced correspondingly. Therefore, even when the load pressure of any one actuator reaches the setting pressure of the main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the target compensated differential pressure is avoided from increasing beyond the target differential pressure in the load sensing control, thus resulting in that the other actuators are not sped up and good operability in the combined operation is ensured.
(3) Further, to achieve the above second object, according to the present invention, the modification value in the above (1) is equal to or smaller than the target differential pressure in the load sensing control set as a variable value depending on the revolution speed of the engine.
With that feature, when the engine revolution speed is lowered and the target differential pressure in the load sensing control, which is set as the variable value depending on the engine revolution speed, is reduced, the modification value set as the target compensated differential pressure is also reduced correspondingly. Therefore, even when the load pressure of any one actuator reaches the setting pressure of the main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the target compensated differential pressure is avoided from increasing beyond the target differential pressure in the load sensing control, thus resulting in that the other actuators are not sped up and good operability in the combined operation is ensured.
(4) In the above (1), preferably, the target compensated differential pressure modifying means includes a signal pressure relief valve which is provided in a maximum load pressure line for detecting the maximum load pressure, and which reduces an upper limit of the maximum load pressure detected by the maximum load pressure line to be lower than the setting pressure of the main relief valve by the modification value.
With that feature, when the delivery pressure of the hydraulic pump rises up to the setting pressure of the main relief valve, the maximum load pressure detected as a signal pressure by the maximum load pressure line is reduced to be lower than the setting pressure of the main relief valve by the modification value. Accordingly, the modification value set as the target compensated differential pressure becomes different from the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators.
(5) Still further, to achieve the above second object, according to the present invention, the signal pressure relief valve in the above (4) is a variable relief valve, and assuming a relief setting pressure of the variable relief valve to be PLMAX0, the target differential pressure in the load sensing control to be PGR, and the setting pressure of the main relief valve to be PR, the relief setting pressure PLMAX0 of the variable relief valve is set so as to satisfy:
PLMAX0=PRxe2x88x92PGR+xcex1
(where xcex1 is a value smaller than PGR)
With that feature, the modification value set as the target compensated differential pressure by the target compensated differential pressure modifying means is provided by PRxe2x88x92PLMAX0=PGRxe2x88x92xcex1, which has a value smaller than PGR (i.e., the target differential pressure in the load sensing control set as a variable value depending on the revolution speed of the engine). Accordingly, as mentioned in the above (3), even when the load-pressure of any one actuator reaches the setting pressure of the main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the target compensated differential pressure is avoided from increasing beyond the target differential pressure in the load sensing control, thus resulting in that the other actuators are not sped up and good operability in the combined operation is ensured.
Also, by setting the modification value set as the target compensated differential pressure to not PGR, but PGRxe2x88x92xcex1 that is smaller than PGR, it is possible to stably perform the load sensing control by the pump control means using a signal pressure corresponding to the same relief setting pressure PLMAX0, and to improve stability of the system.
(6) Still further, to achieve the above second object, according to the present invention, the target compensated differential pressure modifying means in the above (1) includes a selector valve for changing over the target compensated differential pressure from the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators to the target differential pressure in the load sensing control, immediately before the delivery pressure of the hydraulic pump rises up to the setting pressure of the main relief valve.
With that feature, when the delivery pressure of the hydraulic pump rises up to the setting pressure of the main relief valve, the target differential pressure in the load sensing control is set as the target compensated differential pressure (modification value). Accordingly, as mentioned in the above (3), even when the load pressure of any one actuator reaches the setting pressure of the main relief valve during the combined operation in which a plurality of actuators are simultaneously driven, the target compensated differential pressure is avoided from increasing beyond the target differential pressure in the load sensing control, thus resulting in that the other actuators are not sped up and good operability in the combined operation is ensured.
Also, by changing over the signal pressure using the selector valve, the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators can be employed in the load sensing control by the pump control means after the relief. It is hence possible to stably perform the load sensing control and to improve stability of the system.